1. Field of the Invention
This invention relates to end-pumped solid-state lasers and amplifiers, and more particularly to an optical system that has performance which is substantially independent of the polarization or wavelength of the pump beam.
2. Description of Related Art
Laser action generally requires a laser active material and means of pumping said medium. Of particular importance are the solid-state lasers, in which the gain medium is solid; typically a crystal or glass doped with laser-active ions, and the material is optically pumped with light of a suitable wavelength.
Solid-state gain materials often show polarization dependent absorption of the pump light, which is particularly important for polarized pump beams, like those obtained from other lasers, e.g. laser diodes. Often the delivery optics for the pump beam can be simplified if the light entering the crystal is allowed to be unpolarized. An example of this is fiber-coupled pumping using long optical fibers for pump-beam delivery. Moreover, polarization multiplexing is an attractive approach for increasing the brightness of a pump beam by combining two orthogonally polarized pump beams into one. In some cases, including most thin disk laser systems, the pump light will not be absorbed in a single pass, but it will pass the laser crystal multiple times, such that the polarization of the pump light is rotated between individual passes. The integrated polarization of the pump light incident on the crystal can therefore be essentially unpolarized.
However, solid-state lasers are often designed such that they require linearly polarized pump light, e.g. because the laser crystal is cut at Brewster's angle or because its absorption spectrum is strongly dependent on polarization. The latter case usually originates from the desire to use the wavelength and polarization of the strongest absorption of the gain material. For practical lasers, however, this often requires very tight control of the wavelength of the pump light, which implies a control mechanism to adjust the temperature of the pumping laser diodes.
Solid-state lasers are usually pumped at or close to the wavelength of the peak absorption. Since the absorption peak is spectrally relatively narrow, a wavelength variation of the pump usually has a significant impact on the output of the laser. Even if the crystal can be chosen to be thick enough to absorb all pump light, which is possible for 4-level lasers and even 3- or quasi-3-level lasers provided that the crystal is pumped from both sides, changes in the wavelength of the pump light usually have a significant impact on the location wherein the light is absorbed in the crystal. This changes the pump-induced heating of the crystal, and the thermally induced lensing can change dramatically. Such a change in lensing will cause the output beam from an amplifier to change in size and divergence. The problem is more severe in an oscillator, wherein the lensing can cause the cavity to become geometrically unstable.
There is a need for a diode-pumped laser that can operate with unpolarized pump light. There is a further need for a diode-pumped laser that can operate over a large range of pump wavelengths.